The advantage of acrylic polymers as viscoelastic bases for pressure-sensitive adhesives are well known in the art. U.S. Pat. No. RE 24,906 (Ulrich) cites many examples of these products. Initially, such compositions were made via solution polymerization. However, such methods of polymerization involved the use of large amounts of organic solvents. This was undesirable for both safety and economic reasons. Further, the necessity of an excess of such solvents in order to render the polymers transferable between containers drastically limited the handleability and transportability of the polymers.
Various methods of suspension or emulsion polymerization for copolymer pressure-sensitive adhesives have been disclosed in the art. Emulsion polymerization uses water as the reaction medium, and the polymerization takes place within a micelle which easily dissipates the exotherm due to heat of polymerization. Because water is the solvent, the resulting emulsion is safer to handle.
While emulsion polymerization has these distinct advantages, the energy required to dry the water from coated latex materials is about five times higher than for most solvents. Also, the water portion of a dilute latex constitutes a large amount of excess storage capacity and shipping weight when handling emulsion polymers. Thus, there is considerable interest in producing so-called "high-solids" latexes to alleviate some of these problems. Unfortunately, in many cases achieving high solids also meant achieving high viscosities, so that coating the latexes was difficult and expensive.
Thus, there is ongoing interest in producing a high-solids latex adhesive with low viscosity in order to reduce shipping, handling and storage costs, allow increased productivity in plant equipment, reduce drying time for applied latexes, allow the application of films of any desired thickness in fewer passes, and save energy in the drying of latex coatings.
An additional problem with latex adhesives is their sensitivity to moisture, due to the presence of surfactant required in their production. These adhesives become cloudy and lose adhesion and shear strength on exposure to moisture. Moisture-insensitive latex adhesives are thus a desirable goal, and would find use in such applications as weather-sealing tapes, medical tapes, label stock, etc.
WO 89-12618 (Dec. 28, 1989), assigned to PPG Industries, Inc. describes the preparation of ionic alpha-beta ethylenically unsaturated poly(alkylenoxy) surfactants, available under the tradename Mazon SAM 211. Copolymers made with these surfactants, acrylic and methacrylic ester monomers, and from monoethylenically unsaturated monocarboxylic acids, are described. The polymers are said to have good water resistance. No added hydrophobic oligomeric species are described.
EP 194,881 (Mar. 13, 1986), to Smith & Nephew, describes emulsion polymer adhesives which are particularly adapted for application to skin, wherein a copolymerizable surfactant is included in the reaction mixture. The resultant PSAs are said to have good moisture resistance and to produce surgical tapes which adhere well to human skin under dry and wet conditions and onto sweaty skin. Again, no hydrophobic oligomers are present in the reaction mixtures.
U.S. Pat. Nos. 3,925,442 and 3,983,166, assigned to Kendall Company, both incorporated by reference herein teach copolymerizable surfactants. The use of these surfactants in the emulsion polymerization of acrylate monomers is described. These references do not refer to producing acrylate adhesives nor to added styrene (or any other) oligomer, either before or after polymerization.
JP 03 002293 (Jan. 8, 1991), to Soken Kagaku KK, describes emulsion polymerization of acrylic acid esters of C.sub.4 -C.sub.12 alcohols in the presence of a copolymerizable surfactant, with an added monomer of at least one of (meth)acrylic acid or acrylamide. The resultant adhesive is described as having low viscosity and good wetting properties to release paper etc. No added oligomer is mentioned.
U.S. Pat. No. 5,013,784, assigned to Exxon, discloses emulsion copolymerization of acrylic acids and esters in the presence of a low-molecular weight, partially aromatic hydrocarbon resin to form a pressure-sensitive adhesive with good properties. No copolymerizable surfactants are used or taught.
Japanese Patent 90/80,482, assigned to Kanebo NSC K.K., describes emulsion copolymerization of methyl acrylate, 2-ethylhexyl acrylate and acrylic acid in the presence of a rosin ester tackifier to form an adhesive. Copolymerizable surfactants are not taught.
Japanese Patent JP 76/002,793, assigned to Petroleo Brasileiro S. A.-Petrobras, discusses the use of styrene oligomer as a dispersing agent in the aqueous suspension copolymerization of styrene and acrylonitrile. No end use of the copolymer is discussed.
U.S. Pat. Nos. 4,645,711 and 4,629,663, both assigned to 3M, disclose emulsion copolymerization of acrylate monomers in the presence of a vinyl-unsaturated homopolymerizable emulsifier monomer which is a surfactant having both a hydrophobic and a hydrophilic moiety, at a concentration of 5%-50%, preferably 10%-40%. The mixture is coated, dried and used as a pressure-sensitive adhesive. The preferred emulsifier monomer is a monovalent salt of a styrene sulfonate, such as sodium styrene sulfonate. In U.S. Pat. No. 4,645,711, tackifiers, one of which is an alpha-methylstyrene oligomer, are added to the aqueous dispersion after polymerization.
Likewise, U.S. Pat. No. 3,922,464, Silver, et al., assigned to 3M, discloses the use of vinyl-unsaturated homopolymerizable emulsifier monomers which are surfactants having both a hydrophobic and a hydrophilic moiety in emulsion polymerizations of acrylate monomers. In addition, Silver et al. employ a zwitterionic copolymerizable monomer for improved adhesive properties. The emulsifier monomer is used at a concentration of 0.2 to 5 parts by weight while the zwitterionic monomer is used at 0 to 10 parts by weight. No tackifier or other added oligomer is disclosed or claimed.
A need thus exists for a water-based high-solids moisture-insensitive latex pressure-sensitive adhesive which has a low coating viscosity and which has high shear, high compliance and controlled adhesion build-up. We have found such an adhesive.